The present invention relates to clutches, and more particularly to overrunning or one-way clutches having elements that are capable of independent operation.
Several types of clutches that transmit torque in one direction are well known. Such clutches typically have torque-transmitting elements—rollers, pawls, or sprags—disposed between an inner race and an outer race. For example, FIG. 1 (Prior Art) shows a roller clutch with pockets, which includes a cam surface, formed in the outer race to contain the rollers. FIG. 2 (Prior Art) shows a roller clutch similar to that shown in FIG. 1, but with a cage that retains the springs and rollers. The springs in the roller clutches shown in FIGS. 1 and 2 bias the rollers toward the narrow end (that is, the portion having the smallest radial spacing) of the pockets. The cam surface of a roller clutch may also be formed on the inner race (although such a configuration is not shown in the figures).
Whether the rollers are unphased (that is, operate independently of one another) as shown in FIGS. 1 and 2, or phased (that is, urged in unison by a cage into and out of a torque transmitting position), the rollers lodge between the inner and outer races at a narrow portion of the cam surface to transmit torque in only one relative rotational direction. When the inner and outer races are rotated in the relative opposite direction, the rollers disengage as the races rotate such that no torque, or a negligible amount of torque, is transmitted. As oriented in FIGS. 1 and 2, the outer race will transmit torque to the inner race while the outer race is driven counterclockwise, and will not transmit torque while the outer race is driven clockwise.
The terms “relative rotational direction,” “rotational direction,” and “torque transmitting direction” as employed in the specification and claims refer to relative rotation between the races without regard to whether the inner race or outer race is driven. Even in the unphased examples, the rollers engage substantially simultaneously. Such simultaneous engagement prevents undue stress in the rollers and localized portions of the races, and enables the clutch to transmit torque even if one or even a few of the rollers do not engage.
FIG. 3 (Prior Art) shows a schematic of a ratchet or pawl type clutch, in which a pawl pivots clear of a stop formed on the opposing race during rotation in one direction (that is, in the free-wheeling direction), but catches on the stop to transmit torque in the opposite direction (that is, the torque transmitting direction).
In addition to rollers and pawls, sprags are often employed to transmit torque between the inner and outer races of an overrunning clutch. Sprags are struts that have precisely machined cams at opposing ends that wedge between the races to transmit torque in one relative rotational direction, and that enable the races to freewheel while one race overruns the other or while the races turn in the opposite rotational direction. FIG. 4 (Prior Art) illustrates a single cage sprag clutch, and FIG. 5 (Prior Art) illustrates a double caged sprag clutch.
For a sprag clutch to function properly, the sprags typically must operate in phase, and therefore cages are typically required. Thus, referring to FIG. 5 to illustrate a phased configuration, a conventional sprag clutch 100 includes an inner race 102, an outer race 104, several sprags 106 disposed between the inner race 102 and outer race 104, and a spring 108 that urges the sprags 106 toward an engaged position such that the inner and outer contact surfaces of the sprag maintain contact with the inner and outer races, respectively. Clutch 100 also includes an inner cage 110a and an outer cage 110b, as well as an inner drag clip 112a and an outer drag clip 112b. The cages shown in FIG. 5 hold the sprags in position relative to the races and assure equal spacing and circumferential alignment of the sprags, as well as phased operation. Forms (not shown) placed on the side of the sprags may also be employed to phase their operation without the use of cages.
The paper entitled “Automotive Sprag Clutches—Design and Application,” Society of Automotive Engineers No. 208A (E. A. Ferris) describes the importance of phased operation of sprags, and describes the high failure rate of non-phased clutches subjected to shock loads. In this regard, non-phased clutches are prone to failure at loads well below their static torque capacity. Roll over, which is associated with catastrophic clutch failure, occurs, for example, if a first sprag begins to engage prior to other sprags.
For both phased and unphased configurations, the strut angle is crucial to the design and operation of clutches, especially sprag clutches. The strut angle is formed between a line connecting the contact points of the sprag (or other torque transmitting element, such as a roller) at the cam and/or race and a radial line from the cam and/or race center to either contact point. FIG. 6 (Prior Art) illustrates the strut angle, and identifies the inner strut angle, which is formed at the sprag inner contact point, and the outer strut angle, which is formed at the sprag outer contact point. The strut angle determines the normal and tangential forces experienced by the clutch components while under load. The strut angle is also important for assuring appropriate clutch engagement, especially under adverse conditions such as cold weather, under shock loads, and the like.
In addition to more traditional manufacturing techniques for forming the above clutch components, powder metallurgy today is employed to form some components. Employing powder metallurgy for forming such components generally reduces cost, enhances design flexibility, and enhances ease of manufacturing. Powder metallurgy (“PM”) techniques for forming clutch components typically include atomizing prealloyed steel or ferrous raw materials, blending the powder with components such as graphite, copper, nickel, or ferrophosphorus, injecting the mixture into a die, compacting and shaping the mixture by the application of pressure to form a compact, and ejecting the compact from the die.
The compact is then sintered wherein metallurgical bonds are developed under the influence of heat. The alloying and admixed elements enhance strength and other mechanical properties in the sintered part. According to the particular characteristics desired, secondary operations, such as sizing, coining, repressing, impregnation, infiltration, forging, machining, joining, etc., may be employed on the PM part. The term “net shape” or “net forging” will be employed to refer to a part to which no additional machining or related process are required to meet the desired tolerances common to the particular part. A term employed in the powder metallurgy field is near net PM forging, which indicates that only a relatively small amount of machining is typically required.
Each of the above clutch types, whether formed by a powder metallurgy process or other process, has drawbacks that limit its appeal. Roller clutches often are manufactured from wrought material or fully dense powder—that is, at an approximate minimum density of 7.8 g/cc. Hoop and contact stresses in a roller or sprag clutch typically require powder having a 7.80 g/cc density, which makes them more expensive than a lower density option. Moreover, for high torque ratings, roller clutches often require high alloy steels with fine surface finishes to withstand the sliding and rolling contact fatigue inherent in roller clutch design. Further, the number of rollers is constrained because a small roller diameter relative to the cam radius tends to promote cam fatigue.
Ratchet clutches in automotive applications often are manufactured using relatively high density single or double press powder metal processes, typically at approximately densities of 7.0 to 7.3 g/cc. This lower density often results in savings compared with fully dense roller and sprag clutch races. However, tight tolerances and large race diameters are sometimes required for high torque ratings, and such factors diminish or eliminate the cost savings over competing clutches. Further, because of the impact stress inherent in the ratchet design, the races manufactured out of 7.3 g/cc density pm are prone to fracture, and ratchet clutch's poorly distributed load bearing capability results in excessive wear on mating bearing surfaces.
Sprag clutch components often require tight tolerances to operate adequately. Sprags are often formed form cold-drawn wire and are machined or surface finished after hardening to achieve the precise dimensions necessary for sprags to operate acceptably and in unison. Thus, the machining and other processes that are required to produce parts within the particular tolerances often make sprag clutches more expensive than roller and pawl clutches.
It is generally a goal of the present invention to provide improved clutch and clutch components.